Update MockDiscardablePixelRef to use new SkPixelRef constructor.

This change was missed in commit aed1d74 / codereview 105693010 /
svn @163712.

BUG=

Review URL: https://codereview.chromium.org/106963006

git-svn-id: svn://svn.chromium.org/blink/trunk@164122 bbb929c8-8fbe-4397-9dbb-9b2b20218538